Adeno-associated virus (AAV) is a member of the Parvoviridae, a virus family characterized by a single stranded linear DNA genome and a small icosahedral shaped capsid measuring about 20 nm in diameter. AAV was first described as a contamination of tissue culture grown simian virus 15, a simian adenovirus and was found dependent on adenovirus for measurable replication. This lead to its name, adeno-associated virus, and its classification in the genus Dependovirus (reviewed in Hoggan et al., 1970). AAV is a common contaminant of adenovirus samples and has been isolated from human virus samples (AAV2, AAV3, AAV5), from samples of simian virus-15 infected cells (AAV1, AAV4) as well as from stocks of avian (AAAV) (Bossis and Chiorini, 2003), bovine, canine and ovine adenovirus and laboratory adenovirus type 5 stock (AAV6). DNA spanning the entire rep-cap ORFs of AAV7 and AAV8 was amplified by PCR from heart tissue of rhesus monkeys (Gao et al., 2002). With the exception of AAVs 1 and 6, all cloned AAV isolates appear to be serologically distinct. Nine isolates have been cloned, and recombinant viral stocks have been generated from each isolated virus.
AAV appears to commonly infect humans. 50%-80% of adults in North America are seropositive for AAV. A steep rise in antibody response against AAV 1-3 was observed in the age group between 1-10 years (Blacklow et al., 1968). AAV 2 and 3 were readily isolated from anal and throat specimens from children (Blacklow et al., 1967) whereas isolation from adults was not observed. It appears that AAV spreads primarily in the young population (Hoggan, 1970). Prevalence of antibodies against AAV was found to be similar in Europe, Brazil and Japan, which suggests a global spread of AAV (Erles et al., 1999). Infection with AAV appears to be benign in man and laboratory animals. Currently, no disease has been associated with AAV infections.
AAV2 is the best characterized adeno-associated virus and will be discussed as an AAV prototype. The AAV2 genome consists of a linear single stranded DNA of 4,780 nucleotides. Both polarities of DNA are encapsulated by AAV with equal efficiency. The AAV2 genome contains 2 open reading frames (ORF) named rep and cap. The rep ORF encodes the non-structural proteins that are essential for viral DNA replication, packaging and AAV integration. The cap ORF encodes the capsid proteins. The rep ORF is transcribed from promoters at map units P5 and P19. The rep transcripts contain an intron close to the 3′ end of the rep ORF and can be alternatively spliced. The rep ORF is therefore expressed as 4 partially overlapping proteins, which were termed according to their molecular weight Rep78, 68, 52 and 40. The cap ORF is expressed from a single promoter at P40. By alternative splicing and utilization of an alternative ACG start codon, cap is expressed into the capsid proteins VP1-3 which range in size from 65-86 kDa. VP3 is the most abundant capsid protein and constitutes 80% of the AAV2 capsid. All viral transcripts terminate at a polyA signal at map unit 96.
During a productive AAV2 infection, unspliced mRNAs from the p5 promoter encoding Rep78 are the first detectable viral transcripts. In the course of infection, expression from P5, P19 and P40 increase to 1:3:18 levels respectively. The levels of spliced transcripts increased to 50% for P5, P19 products and 90% of P40 expressed RNA (Mouw and Pintel, 2000).
The AAV2 genome is terminated on both sides by inverted terminal repeats (ITRs) of 145 nucleotides (nt). 125 nt of the ITR constitute a palindrome which contains 2 internal palindromes of 21 nt each. The ITR can fold back on itself to generate a T-shaped hairpin with only 7 non-paired bases. The stem of the ITR contains a Rep binding site (RBS) and a sequence that is site and strand specifically cleaved by Rep—the terminal resolution site (TRS). The ITR is essential for AAV2 genome replication, integration and contains the packaging signals.
The single-stranded AAV2 genome is packaged into a non-enveloped icosahedral shaped capsid of about 20-25 nm diameter. The virion consists of 26% DNA and 74% protein and has a density of 1.41 g/cm3. AAV2 particles are extremely stable and can withstand heating to 60° C. for 1 hour, extreme ph, and extraction with organic solvents.
Rep proteins are involved in almost every step of AAV2 replication including AAV2 genome replication, integration, and packaging. Rep78 and Rep68 possess ATPase, 3′-5′ helicase, ligase and nicking activities and bind specifically to DNA. Rep52 and Rep40 appear to be involved in the encapsidation process and encode ATPase and 3′-5′ helicase activities. Mutational analysis suggests a domain structure for Rep78. The N-terminal 225 aa are involved in DNA binding, DNA nicking and ligation. Rep78 and Rep68 recognize a GCTC repeat motif in the ITR as well as in a linear truncated form of the ITR (Chiorini et al., 1994) with similar efficiencies. Rep78 and Rep68 possess a sequence and strand specific endonuclease activity, which cleaves the ITR at the terminal resolution site (TRS). Rep endonuclease activity is dependent on nucleoside triphosphate hydrolysis and presence of metal cations. Rep 78 and 68 can also bind and cleave single stranded DNA in a NTP independent matter. In addition, Rep78 catalyzes rejoining of single stranded DNA substrates originating from the AAV2 origin of replication—i.e., sequences containing a rep binding and terminal resolution element.
The central region of AAV2 Rep78, which represents the N-terminus of Rep52 and Rep40, contains the ATPase and helicase activities as well as nuclear localization signals. The helicase activity unwinds DNA-DNA and DNA-RNA duplexes, but not RNA-RNA. The ATPase activity is constitutive and independent of a DNA substrate. The C-terminus of Rep78 contains a potential zinc-finger domain and can inhibit the cellular serine/threonine kinase activity of PKA as well as its homolog PRKX by pseudosubstrate inhibition. Rep68 which is translated from a spliced mRNA that encodes the N-terminal 529 amino acids (aa) of Rep78 fused to 7 aa unique for Rep68, doesn't inhibit either PKA or PRKX. In addition to these biochemical activities, Rep can affect intracellular conditions by protein-protein interactions. Rep78 binds to a variety of cellular proteins including transcription factors like SP-1, high-mobility-group non-histone protein 1 (HMG-1) and the oncosuppressor p53. Overexpression of Rep results in pleiotrophic effects. Rep78 disrupts cell cycle progression and inhibits transformation by cellular and viral oncogenes. In susceptible cell lines, overexpression of Rep resulted in apoptosis and cell death. Several of Rep78 activities contribute to cytotoxicity, including its constitutive ATPase activity, interference with cellular gene expression and protein interactions.
The first step of an AAV infection is binding to the cell surface. Receptors and coreceptors for AAV2 include heparan sulfate proteoglycan, fibroblast growth factor receptor-1, and αvβ5 integrins whereas N-linked 2,3-linked sialic acid is required for AAV5 binding and transduction (Walters et al., 2001). In HeLa cells, fluorescently labeled AAV2 particles appear to enter the cell via receptor-mediated endocytosis in clathrin coated pits. More than 60% of bound virus was internalized within 10 min after infection. Labeled AAV particles are observed to have escaped from the endosome, been trafficked via the cytoplasm to the cell nucleus and accumulated perinuclear, before entering the nucleus, probably via nuclear pore complex (NPC). AAV2 particles have been detected in the nucleus, suggesting that uncoating takes place in the nucleus (Bartlett et al., 2000; Sanlioglu et al., 2000). AAV5 is internalized in HeLa cells predominantly by clathrin coated vesicles, but to a lesser degree also in noncoated pits. AAV particles can also be trafficked intercellularly via the Golgi apparatus (Bantel-Schaal et al., 2002). At least partial uncoating of AAV5 was suggested to take place before entering the nucleus since intact AAV5 particles could not be detected in the nucleus (Bantel-Schaal et al., 2002) After uncoating, the single stranded genome is converted into duplex DNA either by leading strand synthesis or annealing of input DNA of opposite polarity. AAV replication takes place within the nucleus.
During a co-infection with a helper virus such as Adenovirus, herpes simplex virus or cytomegalovirus, AAV is capable of an efficient productive replication. The helper functions provided by Adenovirus have been studied in great detail. In human embryonic kidney 293 cells, which constitutively express the Adenovirus E1A and E1B genes, the early Adenovirus gene products of E2A, E4 and VA were found sufficient to allow replication of recombinant AAV. Allen et al. reported that efficient production of rAAV is possible in 293 cells transfected with only an E4orf6 expression plasmid (Allen et al., 2000). E1A stimulates S phase entry and induces unscheduled DNA synthesis by inactivating the pRB checkpoint at the G1/S border by interaction with pRB family proteins which results in the release of E2F (reviewed in (Ben-Israel and Kleinberger, 2002). This leads to either induction or activation of enzymes involved in nucleotide synthesis and DNA replication. Since unscheduled DNA synthesis is a strong apoptotic signal, anti-apoptotic functions are required. E1B-19k is a Bcl-2 homolog and E1B-55k is a p53 antagonist. Both proteins have anti-apoptotic functions. E4orf6 forms a complex with E1B-55k and results in degradation of p53. It is also reported to cause S-phase arrest (Ben-Israel and Kleinberger, 2002). E2A encodes a single strand DNA binding protein, which appears to be non-essential for DNA replication but effects gene expression (Chang and Shenk, 1990) (Fields 39, 40). The VA transcription unit affects AAV2 RNA stability and translation (Janik et al., 1989). E1A has a more direct effect on AAV2 gene expression. The cellular transcription factor YY-1 binds and inhibits the viral P5 promoter. E1A relieves this transcriptional block. None of the late Ad gene products have been found to be essential for AAV2 replication. The main function of the helper virus appears to be the generation of a cellular environment with active DNA replication machinery and blocked pro-apoptotic functions that allows high-level AAV replication rather than a direct involvement in AAV replication.
While AAV is usually dependent on a helper virus for efficient replication, low level AAV replication was observed under conditions of genotoxic stress (Yakinoglu et al., 1988; Yakobson et al., 1989). AAV DNA replication and particle formation was also observed in differentiating keratinocytes in the absence of helper virus infection (Meyers et al., 2000). This demonstrates that AAV is not defective per se but rather depends on the helper virus to establish the favorable cellular condition and to provide factors for efficient replication
The ability of AAV vectors to infect dividing and non-dividing cells, establish long-term transgene expression, and the lack of pathogenicity has made them attractive for use in gene therapy applications. Lack of cross competition in binding experiments suggests that each AAV serotype may have a distinct mechanism of cell entry. Comparison of the cap ORFs from different serotypes has identified blocks of conserved and divergent sequence, with most of the latter residing on the exterior of the virion, thus explaining the altered tissue tropism among serotypes (19-21, 48, 56). Vectors based on new AAV serotypes may have different host range and different immunological properties, thus allowing for more efficient transduction in certain cell types. In addition, characterization of new serotypes will aid in identifying viral elements required for altered tissue tropism.